Stationkeeping for drillships and mobile offshore drilling units (MODUs) can be accomplished by providing mooring lines made up of chain, wire, synthetics or combinations of two or more. The top end of a typical mooring line is connected to the vessel using fairleads to change the mooring line direction and winches or chain jacks to tension and secure the mooring line. At the seafloor end of the mooring line, an anchor is provided; these typically can be a high-holding power drag embedment anchor, a suction pile, a plate anchor or driven pile.
Alternatively, the vessel can maintain station by dynamic positioning. Dynamic positioning (DP) typically entails the use of the main propulsion system of the vessel in combination with additional thrusters; the systems are controlled by computers and are linked to navigational aids such as GPS. In this way, the vessel can stay on location without the need for mooring lines and is able to move away from danger on a moment's notice.
For arctic conditions where drifting ice can pose a danger even during the summer drilling season, dynamic positioning (DP) would appear to be the stationkeeping method of choice. Experience has shown, however, that DP will not perform adequately in ice conditions. This leaves mooring systems for stationkeeping which are relatively slow to connect or disconnect; typically on the order of days to complete.
There are emergency release systems available on the market, but these systems simply drop the mooring system onto the seafloor. Re-connecting is therefore quite slow and can be complicated by fouled mooring chain and wire.